1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle which turns steerable wheels in accordance with steering operation imparted to a steering wheel by a driver.
2. Description of the Related Art
Conventionally, there has been known a steer-by-wire-type steering apparatus for a vehicle in which a steering wheel is mechanically separated from the steerable wheels of the vehicle, and when a driver applies a steering operation to the steering wheel, a turning actuator device provided on the steerable wheel side is electrically controlled so as to turn the steerable wheels in accordance with the steering operation applied to the steering wheel. As shown in, for example, Japanese Patent Application Laid-Open (kokai) No. 2002-145098, such a steering apparatus is designed so as to cope with possible cases where the turning actuator device fails to turn the steerable wheels. Specifically, a steering transmission mechanism for mechanically connecting the steering wheel and the steerable wheels is provided; and an electromagnetic clutch is interposed in the steering transmission mechanism so as to establish or break the mechanical connection. With the electromagnetic clutch maintained in a disengaged state or disconnected state, steering of the steerable wheels by means of the turning actuator device is enabled (steer-by-wire mode). When the turning actuator device becomes unable to steer the steerable wheels, the electromagnetic clutch is switched to an engaged state or connected state to thereby mechanically transmit the steering operation from the steering wheel to the steerable wheels, whereby the steerable wheels are turned in accordance with the steering operation applied to the steering wheel (steer-by-mechanical connection mode).
Further, in the steering apparatus disclosed in Japanese Patent Application Laid-Open No. 2002-145098, two turning actuators are provided. When one turning actuator becomes unable to steer the steerable wheels, the electromagnetic clutch is switched to a connected state, and simultaneously, operation of the other turning actuator is switched to assist the turning of the steerable wheels performed in accordance with the steering operation applied to the steering wheel.
However, in the conventional apparatus, the turning characteristic of the steerable wheels in relation to the steering operation in the steer-by-wire mode in which the steerable wheels are turned by means of the turning actuator differs from the turning characteristic of the steerable wheels in relation to the steering operation applied to the steering wheel in the steer-by-mechanical connection mode in which the steering wheel is mechanically connected to the steerable wheels via the steering transmission mechanism. For example, in the steer-by-wire mode, the turning characteristic is typically set such that a driver can steer the steerable wheels to a large angle without changing the positions of his hands on the steering wheel. Further, in the steer-by-wire mode, through utilization of the condition where the steering wheel is not mechanically connected to the steerable wheels, the turning characteristic is changed in accordance with vehicle speed so as to improve steering performance and traveling stability, which cannot be realized in the steer-by-mechanical connection mode. Therefore, in the case where the steer-by-wire mode is switched to the steer-by-mechanical connection mode when turning of the steerable wheels by means of the turning actuator becomes impossible, the change in the turning characteristic gives the driver an unnatural sensation, and the steering stability of the vehicle may deteriorate as a result of the driver's improper steering operation.
Moreover, Japanese Patent Application Laid-Open (kokai) No. 2001-301639 discloses a steer-by-wire-type steering apparatus for a vehicle in which a connection-disconnection apparatus composed of first arid second connection-disconnection units connected in series and each selectively switched between a disconnected state and a connected state is interposed in a steering transmission mechanism. However, this publication does not disclose a method of coping with anomaly of the connection-disconnection units.